1. Field of Invention
This invention relates generally to an improvement in diagnostic medical ultrasound imaging, and more specifically to more effective medical imaging of a human or animal due to improved utilization of an ultrasound contrast agent.
2. Description of the Prior Art
A major goal of contrast agent imaging in diagnostic medical ultrasound is the measurement of physiological functions, such as perfusion of blood into tissue, by measuring the flow of contrast agent into tissue. In order to accomplish this, it is important to differentiate contrast agent from tissue.
One method for achieving the differentiation of contrast agent and tissue is the loss-of-correlation (LOC) method in which multiple pulses are fired into a body containing contrast agent. The first pulse produces a return signal from both contrast agent and tissue, and simultaneously destroys contrast agent. Since contrast agent is destroyed, the second pulse produces relatively more signal from tissue. By subtracting the two signals, the signal from the contrast agent is enhanced; while the signal from tissue, which is in both pulses, is suppressed. The two pulses can be acquired in the same frame (e.g. color Doppler processing) or in different frames. The more contrast agent that is destroyed between firings, the better the differentiation between contrast agent and tissue.
This method has drawbacks. First, a single pulse will rarely destroy all the contrast agent along a given ultrasound line. As a result, many pulses are needed to destroy the contrast agent. Firing many pulses along the same line will reduce the system frame rate, and thereby increase motion artifacts due to tissue motion, especially in cardiac applications. The system frame rate is defined as the actual frame rate at which images can be obtained by the ultrasound system. Secondly, the pulses which best destroy contrast agent are low frequency, long duration (low bandwidth) pulses. However, for better imaging resolution, high frequency, short duration (high bandwidth) pulses are needed. For second harmonic imaging, low frequency pulses are used, but these pulses are still short duration in order to provide good imaging resolution. Usually, a compromise is made between the requirements for imaging and destruction of contrast agent, further reducing the efficiency of contrast agent destruction by ultrasound pulses.
Another approach to LOC imaging, which overcomes this compromise, is to generate two types of pulses, one type for imaging and one type for destroying contrast agent. For example, a transmitter sends an imaging pulse into a region of interest in a body, followed by a pulse for destruction of contrast agent. A second imaging pulse is then sent into the region of interest in the body and the two images of the region before and after contrast agent destruction are compared.
However, this does not overcome the system frame rate problem, but in fact may make it worse. Furthermore, this approach to LOC imaging does not allow contrast agent to perfuse into a region of interest between firings of pulses for destroying contrast agent, so there is little contrast agent to image after destruction. Contrast agent perfusion imaging requires sending the pulses for destroying contrast agent only at specific intervals. Still further, a single pulse will rarely destroy all the contrast agent along a given ultrasound line.
Another approach discussed in the prior art is to use high power pulses simultaneously for imaging and destruction. In this case, low power pulses, or xe2x80x9clocator framesxe2x80x9d are used to image the body, and then at times determined by a timer or an ECG signal, a high power signal is sent to image the contrast agent as well as destroy it. The problem with this approach is that the same high power pulses are simultaneously used to both image and destroy the contrast agent. Therefore, trade-offs in the pulse parameters are made based on the desired amount of destruction, spatial resolution, and desired system frame rate.
Therefore, it would be desirable to improve the LOC method by enhancing the difference between contrast agent and tissue, without sacrificing the system frame rate or imaging resolution. Furthermore, an improved method for measuring physiology, such as perfusion, is needed.
One object of the invention is to enhance the difference between contrast agent and tissue without sacrificing the system frame rate or imaging resolution.
Another object of the invention is to improve the efficiency of contrast agent destruction.
Another object of the invention is to provide a better method for measuring and/or displaying physiology, such as perfusion.
A first aspect of the invention is directed to destroying contrast agent by use of high pulse repetition frequency (HPRF) destruction pulses. These pulses are transmitted at a rate faster than required to allow the pulses to propagate to the farthest boundary of the region of interest being imaged and return to the transducer.
A second aspect of the invention is directed to transmitting multiple destruction beams simultaneously in different directions to destroy contrast agent quickly over a region.
A third aspect of the invention is directed to triggering destruction frames at a fixed point of a physiological signal (e.g., every n beats of a cardiac cycle, where n is an integer), and displaying continuous imaging frames between the destruction frames.
A fourth aspect of the invention is directed to triggering destruction frames at a fixed point of a physiological signal, and displaying, between the destruction frames, imaging frames that are triggered at a different fixed point of the physiological signal.
A fifth aspect of the invention is directed to using triggered destruction frames and imaging frames and comparing imaging frames.
A sixth aspect of this invention relates to combining destruction pulses with subharmonic imaging.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.